Solid-state x-ray imagers using CCD or CMOS devices for dental and other medical applications are well known. Typically, x-ray sensors such as intra-oral dental sensors include a scintillator to convert x-ray photons from the x-ray source into light, which is then imaged by the sensor.
Usually there is no direct connection between a source of x-rays and the imaging apparatus irradiated by the x-rays passing from the x-ray source through or past an object to be imaged by which imaging may be initiated at a beginning of an x-ray exposure. Synchronization of image capture with x-ray emission may therefore be by a trigger mechanism within the sensor.
A number of methods of such triggering are well known. A first comprises continuously monitoring a dark current of some or all imaging pixels of the sensor, comparing the current with a threshold value adjusted for ambient conditions and mapped sensor imperfections such as bright pixels and triggering when the measured value exceeds the adjusted threshold. Thus GB 2304017 discloses a sensor with a comparator circuit for comparing a signal with a threshold to identify a start and finish of an exposure. A similar technique is disclosed in WO 2001/058148 in which a sensor is continuously monitored with cyclic removal of dark current effects. A problem with this approach is that the imaging pixels are not optimised for rapid triggering and are sensitive to stray light.
An alternative triggering mechanism, disclosed in U.S. Pat. No. 5,887,049, is to provide event detector cells separate from the array imaging cells, which are for example CMOS cells, and located, for example, at a periphery of the sensor array. The event detector cells are triggered by light from a scintillator, which also provides light to the imaging cells, and are therefore not optimised for rapid x-ray triggering and are sensitive to stray light. Four spaced-apart detectors are used to avoid simultaneous occlusion of all the detectors by an object being imaged.
A further triggering mechanism is disclosed in EP 06253831.9 in which optimized trigger cells are located in peripheral rows at opposed edges of an imaging array beneath a scintillator covering the imaging array and are addressed alternately.
Since the imaging array is sensitive to optical wavelengths, it is necessary to make the sensor package light-tight, but it is desirable, for comfort, and for x-ray transmission through a source-facing face of the package, to make the package as thin as possible and conveniently from a polymer material. For aesthetic reasons users use polymers coloured light blue, grey and white rather than black, which would otherwise provide the best light exclusion. It is therefore difficult to make the package completely light-tight and false triggering may occur.
In order to be compatible with x-ray exposures as short as 10 ms, a fast response time, of the order of 1 ms, is required for a trigger mechanism to sense an x-ray flux and cause the imaging device to enter an integration mode. Using CMOS detector devices there is no charge binning as with CCD devices, and so signal levels resulting from 1 ms of x-rays are relatively low. In principle it would be possible to average the voltage across each pixel but it is desirable to integrate the trigger mechanism in the sensor device and an averaging function requires too much logic to be readily integrated.